1. Field of the Invention
The invention relates to cross polarization transmission in which signals are transmitted through two polarizations orthogonal to each other, and more particularly to a cross polarization interference canceller for canceling cross polarization interference in those two polarizations.
2. Description of the Related Art
A digital microwave communication equipment is generally designed to operate in two-polarizations transmission in which two polarizations, that is, vertical polarization and horizontal polarization having polarization planes orthogonal to each other, are used in order to enhance an efficiency at which frequencies are utilized. In the two-polarizations transmission, since vertical and horizontal polarizations use a common frequency, if the two polarization planes in an antenna or a space are not orthogonal to each other, signals leak into horizontal polarization from vertical polarization and vice versa.
Such signal leakage is called cross polarization interference, which exerts a harmful influence on transmission quality of signals. In particular, when the above-mentioned two-polarizations transmission and multi-value modulation and demodulation such as QAM are both used, they are significantly harmfully influenced by the cross polarization interference. Consequently, a cross polarization interference canceller (XPIC) is generally used for removing interference.
For instance, Japanese Unexamined Patent Publication No. 2000-165339 has suggested an example of a cross polarization interference canceller.
In a cross polarization interference canceller, polarization or polarized wave which is to be compensated for by canceling the cross polarization interference is defined as self-polarization or self-polarized wave, and a wave orthogonal to the self-polarized wave is defined as other-polarization or other-polarized wave. In order to cancel the cross polarization interference caused by the other-polarization, at a receiver, a relation in phase between a cross polarization interference cancel reference signal transmitted from the other-polarization and the self-polarization signal has to be identical with a relation in phase between the self- and other-polarizations at RF stage where the other-polarization interferes with the self-polarization.
In order to determine a local oscillation frequency of both the self- and other-polarizations to satisfy the above-mentioned requirement, either signal transmission local synchronization or signal receipt local synchronization may be selected.
The receipt local synchronization is often used when semi-synchronization is used for reproducing a carrier signal in a demodulator, because the semi-synchronization having a hardware structure is suitable to reproduction of a carrier signal. The receipt local synchronization is disclosed, for instance, in Japanese Unexamined Patent Publication No. 63-222534.
The receipt local synchronization is grouped into a common local system in which a receiver for the vertical polarization and a receiver for the horizontal polarization have a single local oscillator, and a common reference system in which each of a receiver for the vertical polarization and a receiver for the horizontal polarization has a local oscillator separately from each other, and commonly use a reference signal of the local oscillator.
The common local system has an advantage that a circuit structure thereof can be simplified, but is accompanied with a problem that signals associated with the vertical and horizontal polarizations are concurrently interrupted when a failure occurs in the local oscillator. Though the common reference system is complicated with respect to a circuit structure thereof, a failure in the local oscillator would exert a harmful influence only to one of the signals associated with the vertical and horizontal polarizations.
Thus, when a high efficiency at which radio-waves are utilized is required, and only one of the vertical and horizontal polarizations can be used as a backup line in radio-frequency, the reference synchronization is often selected for ensuring high redundancy.
FIG. 1 is a block diagram of a conventional demodulation system in accordance with the common reference synchronization which is one of the receipt local synchronization.
The illustrated demodulation system is comprised of first and second antennas 14 and 14a, first and second polarization receivers 15 and 15a, first and second local oscillators 16 and 16a, a common reference oscillator 17, and first and second demodulators 18 and 18a. 
Each of the first and second demodulators 18 and 18a is comprised of a primary carrier oscillator 21, first and second multipliers 22 and 22a, first and second low pass filters 23 and 23a, first and second analog-digital converts 24 and 24a, a demodulator unit 35, an adder 28, a judgment circuit 29, and a cross polarization interference canceller 36.
In operation, data signals associated with vertical and horizontal polarizations are input into first and second modulators 11 and 11a through first and second terminals 1 and 1a, respectively. The modulated IF signals are frequency-converted into RF signals in first and second transmitters 12 and 12a, and then, radiated through first and second antennas 13 and 13a. 
A first signal 100 having been transmitted through the first transmitter 12 associated with the vertical polarization, and an interference signal 101a having been transmitted through the second transmitter 12a associated with the horizontal polarization and having leaked into the vertical polarization are merged to each other, and are received in the first antenna 14 as a vertical polarization signal. The vertical polarization signal is frequency-converted into a vertical polarization IF signal 102 in the first receiver 15 which receives a local signal transmitted from the first local oscillator 16 which is in synchronization with the common reference oscillator 17.
Similarly to the above-mentioned case, a second signal 100a having been transmitted through the second transmitter 12a associated with the horizontal polarization, and an interference signal 101 having been transmitted through the first transmitter 12 associated with the vertical polarization and having leaked into the horizontal polarization are merged to each other, and are received in the second antenna 14a as a horizontal polarization signal. The horizontal polarization signal is frequency-converted into a horizontal polarization IF signal 102a in the second receiver 15a which receives a local signal transmitted from the second local oscillator 16a which is in synchronization with the common reference oscillator 17.
In demodulation of the vertical polarization, the first demodulator 18 demodulates the vertical polarization IF signal 102, cancels the cross polarization interference from the vertical polarization IF signal 102, using the horizontal polarization IF signal 102a as a cross polarization interference cancel reference signal, and outputs the thus demodulated data signal from which the interference was removed, through a first terminal 2.
Similarly, in demodulation of the horizontal polarization, the second demodulator 18a demodulates the horizontal polarization IF signal 102a, cancels the cross polarization interference from the horizontal polarization IF signal 102a, using the vertical polarization IF signal 102 as a cross polarization interference cancel reference signal, and outputs the thus demodulated data signal from which the interference was removed, through a second terminal 2a. 
FIG. 2 is a block diagram of the first demodulator 18. Since the second demodulator 18a has the same structure as the structure of the first demodulator 18, only the structure of the first demodulator 18 is explained hereinbelow with reference to FIG. 2.
As illustrated in FIG. 2, the demodulator 35 is comprised of a numerical controlled oscillator (NCO) 25, a first endless phase shifter (EPS) 26, and a carrier synchronization controller 27, and the cross polarization interference canceller 36 is comprised of a second endless phase shifter (EPS) 26a, a transversal filter 30, and a tap coefficient control circuit 31.
An IF signal associated with the self-polarization and input through a first terminal 3 is frequency-converted in the first multiplier 22 by virtue of a primary carrier signal 111 transmitted from the primary carrier oscillator 21. Then, high frequency parts are removed from the IF signal in the first low-pass filter 23, and thereafter, converted into a digital signal by being quantized in the first analog-digital converter 24.
The digital signal and a secondary carrier signal 112 transmitted from the numerical controlled oscillator 25 are both input into the first endless phase shifter 26 for frequency conversion to thereby demodulate a base band signal.
The carrier synchronization controller 27 produces a phase control signal, based on an error signal transmitted from the judgment circuit 29, and controls a frequency of the secondary carrier signal 112 transmitted from the numerical controlled oscillator 25.
An IF signal associated with the other-polarization and input through a second terminal 4 is frequency-converted in the second multiplier 22a by virtue of the primary carrier signal 111 transmitted from the primary carrier oscillator 21. Then, high frequency parts are removed from the IF signal in the second low-pass filter 23a, and thereafter, converted into a digital signal by being quantized in the second analog-digital converter 24a. 
The digital signal and the secondary carrier signal 112 are both input into the second endless phase shifter 26a for frequency conversion to thereby be changed to a cross polarization interference cancel (XPIC) reference signal.
The cross polarization interference cancel (XPIC) reference signal, and a tap coefficient produced in the tap coefficient control circuit 31 for cross polarization interference canceling are both input into the transversal filter 30, in which there is produced a copy signal which reflects interference caused by the other-polarization in a space. The copy signal is removed from the base band signal in the adder 28. Thus, cross polarization interference is removed.
As mentioned above, it is necessary to produce the copy signal which reflects the cross polarization interference caused in a space, by means of the cross polarization interference canceller, in order to cancel the cross polarization interference. To this end, a relation in phase between the self-polarization signal 100 and the other-polarization signal 101 at RF stage where cross polarization interference occurs has to be identical with a relation in phase between a self-polarization base band signal 103 at a base band stage where the interference is removed, and a base band cross polarization interference cancel reference signal 104.
In order to satisfy the above-mentioned requirement, the first and second demodulators 18 and 18a commonly use the primary carrier signal 111, the secondary carrier signal 112, and the reference signal transmitted to the first and second local oscillators 16 and 16a for frequency synchronization in the conventional demodulation system illustrated in FIG. 1. The relation in phase between the first and second demodulators 18 and 18a is compensated for the first and second cross polarization interference cancellers 36 and 36a. 
The cross polarization interference canceling operation in the conventional receipt local synchronization is explained hereinbelow through equations.
Signals are defined as follows.
(A) A base band signal V(t) of the vertical polarization is defined as follows.V(t)=VP(t)+jVQ(t)
VP(t) indicates P-channel parts, and VQ(t) indicates Q-channel parts.
(B) A base band signal H(t) of the horizontal polarization is defined as follows.H(t)=HP(t)+jHQ(t)
HP(t) indicates P-channel parts, and HQ(t) indicates Q-channel parts.
(C) A carrier signal of the vertical polarization is defined as cos(ωVT×t+θVT) wherein “ωVT” indicates a frequency of a carrier signal of the vertical polarization and “θVT” indicates a phase of a carrier signal of the vertical polarization.
(D) A carrier signal of the horizontal polarization is defined as cos(ωHT×t+θHT) wherein “ωHT” indicates a frequency of a carrier signal of the horizontal polarization and “θHT” indicates a phase of a carrier signal of the horizontal polarization.
Under these definitions, RF signals of the vertical and horizontal polarizations VTX(t) and HTX(t) can be expressed as follows.VTX(t)=VP(t)×cos(ωVT×t+θVT)−VQ(t)×sin(ωVT×t+θVT)=real[V(t)×exp(j(ωVT×t+θVT))]HTX(t)=real[H(t)×exp(j(ωHT×t+θHT))]
If it is assumed that the RF signal of the horizontal polarization is merged with the RF signal of the vertical polarization through a coefficient αV, and the RF signal of the vertical polarization is merged with the RF signal of the horizontal polarization through a coefficient αH, the RF signals VRX(t) and HRX(t) of the vertical and horizontal polarizations can be expressed as follows.VRX(t)=real[V(t)×exp(j(ωVT×t+θVT))]+αV×H(t)×exp(j(ωHT×t+θHT))]HRX(t)=real[H(t)×exp(j(ωHT×t+θHT))]+αH×V(t)×exp(j(ωVT×t+θVT))]
Each of the above-mentioned RF signals is frequency-converted in the first and second receivers 15 and 15a in accordance with the following local signals (a) and (b):
(a) Local signal of the vertical polarization: cos(ωR×t+θVR) wherein “ωR” indicates a frequency of the received local signal, and “θVR” indicates a phase of the received local signal of the vertical polarization; and
(b) Local signal of the horizontal polarization: cos(ωR×t+θHR) wherein “θHR” indicates a phase of the received local signal of the horizontal polarization.
Since the local signals are both synchronized commonly with the reference signal, the local signals have the same frequency as each other, and a phase independent of each other.
Herein, it is assumed that the vertical polarization defines self-polarization, which is developed hereinbelow. A received IF signal VIF(t) having passed through a receiver of the self-polarization and a cross polarization interference cancel reference signal VIX(t) having passed a receiver of the other-polarized can be expressed as follows.VIF(t)==real[V(t)×exp(j((ωVT−ωR)×t+(θVT−θVR)))]+αV×H(t)×exp(j((ωHT−ωR)×t+(θHT−θVR)))]VIX(t)==real[H(t)×exp(j((ωHT−ωR)×t+(θHT−θHR)))]+αH×V(t)×exp(j((ωVT−ωR)×t+(θVT−θHR)))]
Since the first demodulator 18 matches an oscillation frequency of its internal oscillator to a frequency and a phase of the received IF signal, a frequency of the received IF signal is equal to an oscillation frequency of the first demodulator 18, as follows.(ωVT−ωR)=(ωD1+ωD2)(θVT−θVR)=(θD1+θD2)
In these equations, “ωD1” indicates a frequency of the received primary carrier signal, “ωD2” indicates a frequency of the received secondary carrier signal, “θD1” indicates a phase of the received primary carrier signal, and “θD2” indicates a phase of the received secondary carrier signal.
A received base band signal VBB(t) resulting from demodulation of the received IF signal is expressed as follows.VBB(t)=real[V(t)×exp(j((ωVT−ωR−ωD1−ωD2)×t+(θVT−θVR−θD1−θD2)))+αV×H(t)×exp(j((ωHT−ωR−ωD1−ωD2)×t+(θHT−θVR−θD1−θD2)))]=V(t)+real[αV×H(t)×exp(j((ωHT−ωVT)×t+(θHT−θVT)))]
A base band cross polarization interference cancel (XPIC) reference signal VBX(t) which is to be input into the transversal filter 30 is expressed as follows.VBX(t)=real[H(t)×exp(j((ωHT−ωR−ωD1−ωD2)×t+(θHT−θHR−θD1−θD2)))+αH×V(t)×exp(j((ωVT−ωR−ωD1−ωD2)×t+(θVT−θHR−θD1−θD2)))]=real[H(t)×exp(j((ωHT−ωVT)×t+(θHT−θHR−θVT+θVR)))+αH×V(t)×exp(j(θVR−θHR))]
Since the cross polarization interference canceller 36 produces a copy signal reflecting the cross polarization interference existing in the received base band signal, based on the cross polarization interference cancel reference signal, the copy signal VXPIC(t) produced by the cross polarization interference canceller 36 and a response signal made by the cross polarization interference canceller 36 are expressed as follows, if the cross polarization interference canceller 36 ideally operates.VXPIC(t)=real[−αV×H(t)×exp(j((ωHT−ωVT)×t+(θHT−θVT)))]−αV×αH×V(t)
The response signal=−αV×exp(j(θHR−θVR)).
The received base band signal and the cross polarization interference cancel reference signal are added to each other in the adder 28, resulting in that the cross polarization interference is removed from the base band signal VO(t), which is expressed as follows.VO(t)=V(t)−αV×αH×V(t)
The second component “αV×αH×V(t)” in the base band signal VO(t) constitutes an interference part between signs. However, since αV and αH are generally much smaller than 1, the second component can be disregarded. If αV and αH cannot be disregarded, the interference part could be removed by additionally using an equalizer.
A local oscillator generally used at RF stage includes much phase noises, and hence, time fluctuation is generated in a phase-relating component, if a frequency-relating component is kept fixed. In the receipt reference synchronization, since the vertical and horizontal polarizations use local oscillators separately from each other, oscillation frequencies of the local oscillators can be made equal to each other by means of a common reference oscillator. However, since phase noises are generated in the local oscillators independently of each other, time fluctuation remains also in local phase difference terms of the vertical and horizontal polarizations.
In order for the cross polarization interference canceller 36 to properly operate under the above-mentioned circumstances, the cross polarization interference canceller 36 is required not only to make a copy of the cross polarization interference part αV, but also to follow a local phase difference exp(j(θHR−θVR)) which varies with the lapse of time.
However, since the cross polarization interference canceller 36 includes the transversal filter 30, the cross polarization interference canceller 36 can make the copy signal reflecting a phase difference which does not vary with the lapse of time, but cannot follow a phase difference rapidly varying with the lapse of time, and hence, cannot make the copy signal reflecting such a phase difference. This is because the transversal filter 30 cannot follow the dynamic characteristics required in the tap coefficient control circuit 27.
In addition, since the ability to follow a phase difference is degraded as a tap coefficient of the transversal filter 30 becomes great, the ability is degraded as the cross polarization interference becomes great.
As having been explained, the conventional cross polarization interference canceller of the receipt reference synchronization type is accompanied with a problem that the ability of canceling the cross polarization interference is unavoidably degraded, if a phase noise component is increased in the local oscillator.
Japanese Unexamined Patent Publication No. 63-31981 has suggested a circuit for canceling cross polarization, including a first synchronization detector to which an input signal associated with first polarization is input, a second synchronization detector to which an input signal associated with second polarization orthogonal to the first polarization is input, and a reproducer which reproduces interference part. The reproducer is comprised of first means for detecting beats of reference carrier waves of the first and second synchronization detectors, second means for multiplying an output transmitted from the first means, by an output transmitted from the second synchronization detector to thereby produce a first pseudo-interference signal, and third means for multiplying a complex conjugate signal in an output signal transmitted from the first means, by an output signal transmitted from the first synchronization detector to thereby produce a second pseudo-interference signal. The first pseudo-interference signal is used for removing a polarization interference signal included in the output signal transmitted from the first synchronization detector, and the second pseudo-interference signal is used for removing a polarization interference signal included in the output signal transmitted from the second synchronization detector.
Japanese Unexamined Patent Publication No. 3-72732 has suggested a cross polarization interference canceller including first means for receiving horizontal and vertical polarization signals transmitted through polarizations orthogonal to each other, second means for detecting a relation between an error signal indicative of interference which is caused by the vertical polarization signal and which leaked into the horizontal polarization signal, and an identification signal obtained from the vertical polarization signal, and transmitting a first interference cancel signal, third means for removing the interference from the horizontal polarization signal in accordance with the first interference cancel signal, fourth means for detecting a relation between an error signal indicative of interference which is caused by the horizontal polarization signal and which leaked into the vertical polarization signal, and an identification signal obtained from the horizontal polarization signal, and transmitting a second interference cancel signal, fifth means for removing the interference from the vertical polarization signal in accordance with the second interference cancel signal, and sixth means for sampling both one of the polarization signals for obtaining the error signal and the other of the polarization signals for obtaining the identification signal through a common clock signal, and supplying the thus sampled signals to the second and fourth means.
Japanese Patent No. 2669235 (Japanese Unexamined Patent Publication No. 5-211493) has suggested a cross polarization interference canceller which, on receiving primary and secondary polarization signals transmitted through polarizations orthogonal to each other in synchronization with clock signals, removes a secondary polarization signal part having cross-interfered with the primary polarization signal. The cross polarization interference canceller includes first means for transmitting a first reproduction clock signal, based on a first base band signal obtained by demodulating the primary polarization signal, in synchronization with a clock signal of the primary polarization signal, second means for transmitting a second reproduction clock signal, based on a second base band signal obtained by demodulating the secondary polarization signal, in synchronization with a clock signal of the secondary polarization signal, third means for transmitting a third reproduction clock signal, based on the first base band signal, in synchronization with a clock signal of the above-mentioned secondary polarization signal part, fourth means for detecting a phase difference between the second and third reproduction clock signals, and controlling a phase of the first reproduction clock signal in accordance with the phase difference to thereby transmit a fourth reproduction clock signal, fifth means for sampling the first base band signal by virtue of the first reproduction clock signal to thereby produce a first digital signal, sixth means for sampling the second base band signal by virtue of the fourth reproduction clock signal to thereby produce a second digital signal, a transversal filter which transmits a cancel signal having the same frequency and amplitude as those of the secondary polarization signal part, based on the second digital signal, seventh means for delaying the first digital signal by a period of time required for the transversal filter to produce the cancel signal, and eighth means for subtracting the delayed first digital signal from the cancel signal to remove the secondary polarization signal part.
However, the above-mentioned problem remains unsolved even in these Publications.